Conditional pageref for labels far away

(Updated answer after the OP posted a second, more elaborate MWE.)

The varioref package implements the following decision rule with regard to generating page call-outs:

• If the item being cross-referenced and the cross-reference are on the same page, no page-related affix is produced;

• If the item being cross-referenced and the cross-reference are on adjoining pages, either "on the previous page" or "on the following page" is affixed to the cross-reference.

• If the item being cross-referenced and the cross-reference itself are two or more pages apart, a "on page x" affix is generated.

If you know in advance that there will be several pages between the cross-reference and the item being cross-referenced, you may also use the instruction \fullref, which is simpler (uses fewer machine cycles...) than \vref.

If you use both varioref and hyperref, you should load the cleveref package as well: Doing so fixes a (mild) conflict between the former two packages. If you load cleveref, the items' names ("equation", "theorem", etc) will be prefixed automatically to the cross-reference call-outs.

\documentclass{scrreprt}
\usepackage{blindtext}    % lorem ipsum...
\usepackage{amsmath}
\usepackage[amsmath]{ntheorem}

\usepackage{varioref}     % for \vref and \fullref macros
\usepackage[colorlinks]{hyperref}
\usepackage[nameinlink]{cleveref}

\newtheorem{thm}{Theorem}

\begin{document}
\setcounter{chapter}{1} % just for this example

% theorem foo, with cross-reference to an equation
\begin{thm}\label{thm:foo}
The future belongs to those who believe in their dreams (see \vref{eqn:bar}). 
\end{thm}

% some pages of text...
\blindtext[15]

% equation bar, with cross-reference to a theorem 
\begin{align}\label{eqn:bar}
    a+b=c\quad\text{(cf.\ \fullref{thm:foo})}
\end{align}
\end{document}